Semiconductor chips or wafers are used in many applications, including as processor chips for computers, and as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices.
A common circuit component of semiconductor chips is the transistor. In ULSI semiconductor chips, a transistor is established by forming a polysilicon gate on a silicon substrate and separated therefrom by a gate insulator. Source and drain regions are then formed in the substrate beneath the gate by implanting appropriate dopant materials into the areas of the substrate that are to become the source and drain regions. This generally-described structure cooperates to function as a transistor.
Importantly, as mentioned above the gate must be insulated from the source and drain regions by a thin gate insulator, typically a layer of oxide. As the size of the transistors is reduced by ULSI technology, the thickness of the gate insulator becomes ever smaller. Owing to the relatively high electric field across the gate insulator layer, however, charge carriers undesirably can tunnel across the gate insulator layer, particularly when the gate insulator layer is made very thin as it is in ULSI technology. This renders the transistor "leaky", degrading its performance.
To alleviate this problem, high-k dielectrics (dielectrics that have high dielectric constants) are used as the gate insulator. Unfortunately, undesirable chemical reactions can occur between the high-k gate insulator and the conventional polysilicon gate electrode above the gate insulator. For this reason, it can be advantageous to use metal gate electrode materials in place of or in addition to the polysilicon when high-k material is used as the gate insulator.
The present invention recognizes, however, that a metal electrode that is sufficient for establishing a gate of a P-channel MOSFET might not be sufficient for establishing a gate of an N-channel MOSFET. Specifically, as understood by the present invention a metal electrode that satisfies the threshold voltage design for one type of MOSFET might not satisfy the threshold voltage design for the opposite type of MOSFET or for the same type of MOSFET having a different threshold voltage requirement. As further recognized by the present invention, greater flexibility in designing and implementing MOSFET circuits on ULSI devices would be provided if a single ULSI semiconductor chip were able to incorporate high-K dielectric gate insulators and the metal gate electrodes they entail, and also support more than one threshold voltage. Fortunately, the present invention addresses this consideration.